output of ε and μ tensor components

[stevengj]

Currently, h5fields.cpp, integrate.cpp, array_slice.cpp and so on only allow you output the average eigenvalue of the ε and μ matrices. It would be nice to be able to get/output any desired component of the tensor (see e.g. #627).

That would require implementing 10 new component values (Dielectric_xx, Dielectric_xy, etcetera) and going through all of the places that handle Dielectric and adding the requisite support. Straightforward but somewhat arduous.